Yes the charges were prebagged and placed in powder tanks within the magazine. Tanks were to be marked with size of charge. An empty tank was used in each landing party boat as a temp. magazine for the loaded small arms cartridge boxes and spare cartridges. The 1864 ord instructions give the number of charges each tank could hold. The 150lb tank held approx 18 charges for an 8inch/64 pounder gun. The tank was placed on it's side with the hinge down so the charges could be handed out to thru the magazine screens. Tanks capacity was based on pounds of powder in grains thus you make a good point on this maybe being used ashore since the powder charge size isn't painted on the lid. a 150lb tank was 22 1/8 inch tall with 15X15 inch sides. This tank was found in the Cayman Islands.
Geo Dailey
gunnersmate U. S. Naval Landing Party
Navay/Marine Living History Association